Liquid or gaseous feedstock streams, particularly hydrocarbon streams such as natural gas often contain substantial amounts of sulphur compounds, for example, where the hydrocarbon is gaseous, in an excess of 50 ppm by volume expressed as equivalent hydrogen sulphide.
Before use it is generally desirable to reduce the sulphur compounds content of the feedstock to a low level, for example to below 10 ppm by volume.
One method of sulphur compound removal that could be employed is to sorb sulphur compounds from the stream by the use of a regenerable sorbent material such as a molecular sieve, active charcoal, or alumina. In this process the sorption stage is effected at a relatively low, e.g. ambient, temperature, and regeneration of the sorbent can be effected by contacting the regenerable sorbent with a suitable fluid, e.g. part of the feed or product, heated to an elevated temperature, generally in a direction counter-current to the direction of flow when on sorption duty. In such a process after the regeneration stage, there is normally a step wherein the regenerable sorbent is cooled to the sorption temperature, prior to the return of the regenerable sorbent to sorption duty. This cooling step is necessary since the regenerable sorbents are not particularly effective at elevated temperatures and so return of the regenerable sorbent, while still hot, to sorption duty would result in a significant break-through of sulphur compounds into the product while the regenerable sorbent is cooling. This cooling step can be effected by passing part of the product, or feed, at the normal sorption temperature through the regenerable sorbent. Where, as is normal, the regeneration fluid is part of the feed or product stream, the fluid employed for the regeneration (including cooling) stage represents a considerable proportion of the feed or product and has to be sent to waste, and so renders this type of process unattractive economically except in certain specialised cases.
Another method of sulphur compound removal that is commonly employed is to contact the feedstock stream with a bed of particles of a suitable particulate non-regenerable sorbent, such as zinc oxide. While such beds enable a product stream of low sulphur content to be produced, such particulate non-regenerable sorbents have only a limited capacity and so if large quantities of sulphur compounds have to be removed, the beds of particulate non-regenerable sorbents need frequent replenishment.